SSM/I-derived global rainfall statistics and their application

Rainfall information in various part of the globe is needed in climate studies. This information includes the space-time rainfall amount, and it's spatial and temporal pattern. As input to all hydrologic systems rainfall amount and how it behaves in space and time affect various components of the system such as evapotranspiration, sensible heat flux, and runoff. These components, in turn, will feed back on the atmosphere. As important as it is, however, detailed rainfall information is traditionally not available over most part of the globe as provided by surface-based sensors. Space-based remote sensing seems to be the only viable way to provide detailed global rainfall data.

NOAA/NESDIS/ Office of Research and Application Hydrology Team has been developing its SSM/I rainfall algorithm and producing operational global rainfall products using the algorithm. Recently, the type of rainfall products has been expanded to include the following: 2.5-degree latitude-longitude monthly rainfall, the error associated with the monthly rainfall estimates, variance of instantaneous 2.5-degree rain rate and its 4-hour and 8-hour lag correlation, temporal frequency of rain and fractional spatial coverage of rain in a 2.5-degree grid box. These data sets are useful in climate and hydrology models.

One potential application is the use of the spatial rainfall information in the surface hydrology scheme of GCMs. Traditionally, uniform rain rate is assumed over the entire large GCM grid. This assumption has been shown to result in over-estimation of evaporation and corresponding under-estimation of sensible heat flux and surface runoff. Thus it has been urged by many researchers to employ the actual spatial coverage of rain in the surface hydrology scheme, which necessitates a global data set of this parameter. The products described in this paper fill this need.

As a demonstration of the value of the spatial rainfall coverage data set, we perform a simulation using a simple surface hydrology scheme in which the uniform and actual distribution of rainfall are used and the resulting evaporation and surface runoff are compared.